i965: Fix constant propagation into 32-bit integer MUL.

i965: Fix constant propagation into 32-bit integer MUL.

i965's MUL instruction can't take an immediate value as its first
argument.  So normally, if constant propagation wants to propagate a
constant into the first argument of a MUL instruction, it swaps the
order of the two arguments.

This doesn't work for 32-bit integer (and unsigned integer)
multiplies, because the MUL operation is asymmetric in that case (it
multiplies 16 bits of one operand by 32 bits of the other).

Fixes piglit tests {vs,fs}-multiply-const-{ivec4,uvec4}.

Reviewed-by: Eric Anholt <eric@anholt.net>